Download - Module 8 - Metamorphic Rocks

7/30/2019 Module 8 - Metamorphic Rocks

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Module 8Metamorphic Rocks


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Metamorphic rocks are changed rocks

The proto l i th(the parent roc k) is thepreexisting rock from which themetamorphic rock was formed

They are formed in the sol id stateinresponse to the following pr inc ipal

agents of metamorphism:

Change in pressure Change in temperature

Change in pressureand temperature

METAMORPHIC ROCKS


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Metamorphism may cause a change in arocks:

Structure/Texture

Mineral assemblage

Compos i t ion (s l ight ly)

Al l of the above

METAMORPHIC ROCKS


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1. Protolith Composition2. Pressure

3. Temperature

4. Time

Facto rs That Contro l the Character ist ics ofMetamorph ic Rock s


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1. Protolith Composition

Mafic protoliths (basalt, diabase, and gabbro)

yield a dark charcoal gray mafic metamorphic

rock called amphibol i te

Felsic protoliths (granite, rhyolite, mudstone,

sandstone, conglomerate, breccia) yield light

colored tan, silver, light to medium gray, etc.

felsic metamorphic rocks



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2. Pressure (stress) Increasing pressure flattens grains, crushes

grains (reduces size), causes shearing

Confining pressure, associated with depth ofburial, is equal in all directions

Differential pressure is not equal in alldirections

Produces foliation (parallel alignment ofgrains)

Pure shear = compression, flattens grains Simple shear = skewing, stretches grains,

produces lineation, aligns elongate grainsin direction of transport



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Conf in ing Pressure

Pressure is equal in

all directions



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Differential Pressu re Differential pressure

Is notequal in all directions

Leads to format ion of fo l iat ionby either

Pure shear (f lattening)

Simp le shear (skew ing )

Pure shear (f lattening) Simp le shear (skew ing )



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Type of foliation

Slaty Cleavage Alignment of small mica

flakes

Schistos i ty Alignment of large micaflakes

Gneiss ic Band ing segregation of felsic

and mafic minerals into alternating lightand dark bands



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Bedding is perpendicular to cleavage

at the top and bottom of the fold

Fol iat ion : Developm ent o f Slaty Cleavage

Facto rs That Contro l the Character ist ics ofMetamorphic Rocks


12/32http://www.tulane.edu/~sanelson/geol212/contactmeta.htm

Fol iation: Development o f Schisto s i ty



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Fol iation: Format ion o f Gneiss ic Banding


F Th C l h Ch i i f


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3. Temperature Inc reasing temperaturecauses increased

movement of ions which in turn causes

Mineral grains to grow larger

Minerals to recrystallize The upper limit on T for metamorphism is

partial melting


4. Time Metamorphic reactions and textural changes

require millions of years to occur


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Metamorphic Facies

o

Cl i f i t i f M t hi R k


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Class i f icat ion of Metamorphic Rocks

1. Regional metamorphic rocks Form in response to increased temperature

and directed pressure along plate boundaries

Are foliated

Were sheared by pure shear or simple shear

2. Local metamorphic rocks

Form in response to contact with magma athigh T and /or high confining P

Found adjacent to igneous intrusions

Are usually unfoliated


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Where do the metamorphic rocks form?


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Contact metamorphic rocksRegional metamorphic rocks

Class i f icat ion of Metamorphic Rocks



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Class i f icat ion of Metamorphic RocksFormat ion of Contact Metamorph ic Rocks

Proto l i th and Metamorphic Rocks


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Proto l i th and Metamorphic Rocks

T f M t h i R k


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Are formed by differential pressure (pure shear, simple shear)

Slate

Dull, microscopic grains, strong slaty cleavage, anycolor, mudstone protolith

Phyllite

Shiny, strongly micaceous with microscopic grains,strong schistosity, cleavage, can be mafic or felsic, anyrock-type protolith

SchistShiny, strongly micaceous, fine to medium grained,strong schistosity, cleavage, any color, any rock-typeprotolith

Gneiss

Dull, weakly micaceous, fine to coarse grained, banded,

weak cleavage, any rock-type protolith, but most oftensheared coarse grained protolith such as granite,conglomerate, breccia

Migmatite

Dull, weakly micaceous, fine to coarse grained, foldedbanded texture, weak cleavage, almost melted, any rock-

type protolith

A. Fol iated Metamorphic Rocks

Types o f Metamorph ic Rocks

T f M t h i R k


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Protol i th:Fine grained rock like

shale, mudstone, or siltstone

Slate

Appearance:Dull, microscopic

grains, strong slaty cleavage,

any color


T f M t h i R k


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Phyl l i te

Appearance:Shiny, strongly

micaceous with microscop ic

grains, has strong schistose

cleavage, can be felsic

(muscovite-rich, silvery) orMafic (biotite-rich, shiny

medium to dark gray)

Protol i th:Can be any rock-type


T f M t h i R k


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Schist

Appearance:Shiny, strongly

micaceous, fine to medium grained

has strong schistose cleavage,

can be felsic (muscovite-rich,

silvery) or mafic (biotite-rich,shiny medium to dark gray)




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Migmat i te

Appearance:Dull, weakly micaceous,

fine to coarse grained, folded banded

texture, weak cleavage, almost melted,

a folded gneiss



Types o f Metamorphic Rocks


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Quartzite

Metamorphosed sandstone

Harder than marble

Will not fizz (release CO2) in the presence of acid

May be fol iated o r un fol iated

B. Special Types of Metamo rph ic Rocks


http://resourcescommittee.house.gov/subcommittees/emr/usgsweb/photogallery/images/Quartzite%202_jpg.jpg


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B. Special Types of Metamorphic Rocks

Marble

Metamorphosed limestone

Contains calcium carbonate

Will fizz (release CO2) in the presence of acid Softer than quartzite



http://www.pitt.edu/~cejones/GeoImages/6MetamorphicRocks/Marble/MarbleWhite.jpg


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Anthracite

Metamorphosed bituminous coal

Low density

Very shiny black




http://resourcescommittee.house.gov/subcommittees/emr/usgsweb/photogallery/images/Coal,%20anthracite_jpg.jpg


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Amphibolite

Dull black, fine to coarse grained, weak cleavage ifsheared (gneissic)

Metamorphosed gabbro, basalt, or diorite

Dark rock composed mostly of amphibole and Ca-Na plagioclase, may be

either foliated or unfoliated, often with large (visible) elongated crystals of

amphibole



http://www.smccd.net/accounts/hand/geology101/metamorphic/images/amphibolite2.jpghttp://www.smccd.net/accounts/hand/geology101/metamorphic/images/amphibolite1.jpghttp://www.smccd.net/accounts/hand/geology101/metamorphic/images/amphibolite2.jpghttp://www.smccd.net/accounts/hand/geology101/metamorphic/images/amphibolite1.jpg